This invention relates to seismic data tape recording systems, and, more particularly, to means for increasing the serial digital data storage capacity of magnetic tape.
It is customary in seismic geophysical surveying systems to store the acquired seismic data on magnetic recording tape. The amount of tape required to store the data has increased as more seismometers and groups of seismometers have been used to sense seismic signals. The use of mechanical vibrators to generate swept frequency acoustic waves in the ground has also increased the amount of data to be stored. The swept frequency signals are of longer time duration than the impulse-type signals generated by, for example, dynamite. In addition, the swept frequency signals are often repeated 10 or more times to provide signals which are summed to improve the signal-to-noise ratio.
Historically, as the amount of seismic data increased, recording systems were simply made larger by adding more recording tracks to tape recorders and by using more recorders and more cables to conduct signals from seismometers to the recorders. Such systems have become so complex that providing direct-wire cabling from all seismometers to the recorders has become impractical.
U.S. Pat. 3,806,864, entitled "Cableless Seismic Digital Recording System," issued to Broding, et al., Apr. 23, 1974, illustrates a solution to several of these problems. In the Broding patent, the cabling problem is solved by providing a plurality of portable tape recorders which are located in the field near the seismometer groups to which they are connected, and the recorders are controlled by radio signals from a centrally located truck. The small lightweight tape recorders used in this type of portable system typically use a narrow magnetic tape, and record only one channel of data. Samples of seismic data which are usually represented by 16 to 18 bit digital words must therefore be recorded serially on such tape recorders. At a recording density of about 800 bits per inch, approximately 6 feet of recording tape are required to record a typical 6-second record after an impulse-type initiation. This is based on a 2-millisecond sample period and 18 bit words describing each sample. For a typical 300-foot magnetic tape reel, which can be recorded in both directions to yield 600 feet of recording, a maximum of 100 records may be stored. In practice, the number of recordings which may be stored on a 300-foot tape reel varies between 50 and 75 due to the amounts of tape used in starting and stopping the recorder, and in recording identifying information about the records. This total recording capacity is sufficient to record a typical day's number of records when only impulse-type sources are used. If vibrator-type sources are used, this same 300-foot tape reel can store the information from only two or three complete records. This is because the record listening period is extended to about 10 seconds, and is repeated typically ten times so that approximately 100 feet of tape are consumed in storing the entire amount of data which constitutes a single record. Due to this fact, the portable, cableless digital recording systems have been limited to use with impulse-type seismic sources.
It is not practical to increase the data storage capacity of the portable recorders by increasing tape length or width for several reasons. Such changes would make the recorders larger and heavier and therefore less portable. Of greater importance is the fact that a larger tape requires more driving power to operate. The portable systems must be battery powered and are therefore inherently power limited. Thus, it is seen that the usefulness of portable recording systems in seismic work is limited by the data storage capacity of magnetic recording tape.
As disclosed in the above-referenced Broding patent, the typical seismic digital recording system includes a gain ranging amplifier having a gain control input driven by a digital gain level signal. This gain control input typically can change the amplifier gain by factors of two or four for each binary unit of input. The output of the gain ranging amplifier is digitized by an analog-to-digital converter which typically provides a twelve bit word, commonly called a "mantissa." The gain control input of the gain ranging amplifier is a four bit word which must be used together with the mantissa to completely define the amplitude of each sample. The purpose of this gain ranging amplifier arrangement is to increase the dynamic range of the digital recording system without greatly increasing the number of bits used to describe the amplitude of each sample.